The present invention is directed to the voltage sources and current sources, particularly those of type employed in equipment for testing analog circuits.
In testing analog circuits--and sometimes in testing digital circuits as well--it is often necessary to drive or draw a predetermined current or apply a predetermined voltage having very specific characteristics. For instance, in order to test the noise rejection of a power supply, it may be desirable to apply a signal that is a combination of a d.c. value and an a.c. component. In order to test the ability of the supply to carry a load, a current source or sink may be used at the output port of the power supply under test to present it with a well-defined load.
When such current or voltage sources are incorporated in automated test equipment, they can be subject to certain problems. For instance, a current source in such equipment must be connected and disconnected frequently to devices under test, and there is a tendency for the current source, which is attempting to drive a very high impedance (an open circuit) just before connection, to generate high transient voltages when the connection occurs. This can be destructive to the device under test, and it is not beneficial to the test equipment, either.
Additionally, certain devices under test have voltage limits that the current source should not exceed. In order to avoid exceeding such limits, limiting devices are often included in the feedback network of the current source, but there is an inherent delay in such arrangements--they can only react to overvoltages at the output, not anticipate them--so the voltage clamping cannot be entirely effective unless elaborate additional circuitry is provided at the output port of the current source.
Finally, the current sources employed in automated test equipment are usually employed with a wide variety of loads. Such sources are feedback devices, and the type of feedback network that will result in a stable operation with one type of load can be highly unstable with other types. Accordingly, it has been necessary in the past to provide many alternative types of feedback networks--and the switching circuitry for choosing among them--in order to deal effectively with different types of loads.
The object of the present invention is to supply current or voltage in a manner in which clamping functions are carried out readily without additional circuitry, in which switching transients are largely eliminated, and in which adjustment of feedback is accomplished without a lot of alternative circuitry and the attendant switching.